This invention relates in general to electro-optical scanning systems. More specifically, it relates to a system which uses scattered laser radiation to detect, measure and locate minute dust particles on the surfaces of a reticle plate.
In the manufacture of microelectronic circuits by photolithography, the lay-out of a circuit is frequently first reduced to a physical form as a pattern of dark areas on a photolithographic mask, commonly termed a reticle. The reticle is placed on the stage of a radiation projector to produce an exposure, usually of reduced size, on a wafer which in turn produces a layer of the circuit. It is extremely important that the reticle be free of dust or other particulate matter on both of its surfaces since the dust will be projected as a dark area in the printing of the wafer. The presence of dust on the reticle will often result in an electrical connection or other circuit modification which causes the microelectronic circuit to malfunction. The error will normally not be detected until after a set of wafer chips have been produced. These circuits must be discarded. The economical production of high quality microelectronic circuits is therefore directly related to the ability to detect and eliminate minute dust particles which may be present on the reticle just before it is seated on the optical stage of the projector.
Previously reticles were inspected for dust prior to replacement in the projector by manual inspection in the open air. This procedure has numerous drawbacks. First, a relatively high level of experience and concentration is required of the person inspecting the reticle. Manual inspection is particularly difficult for extremely minute dust particles, for example those having a diameter of approximately 1 to 5 micrometers. Second, if the visual inspection does detect a dust particle, it is difficult to accurately determine its location to remove the particle. Third, because the inspection is performed in an uncontrolled, open-air environment, the reticle can acquire a dust particle after it has been inspected.
Most applications of radiation scattering to the detection or measurement of particles involve the use of forward scattered light from particles suspended in a fluid. However, in a few instances light scattering techniques have been used to examine solid surfaces. For example, J. F. Ready in Industrial Applications of Lasers at pp. 331-333 describes several systems for surface inspection using laser light and photodetectors. One system detects the presence of gold nodules on a ceramic surface using light scattered from the nodules. U.S. Pat. No. 3,767,306 describes another system where light is scattered from particles immersed in a thin liquid layer covering the surface. The inspection of a glass surface for the presence of extremely small particles such as dust, however, presents unique problems which are not addressed by these prior art systems. One significant difference is that the surface of the glass reticle is itself comparatively rough and therefore scatters light. This glass induced scattering presents a general background noise which can easily overwhelm the scattering induced by a small dust particle. Another distinction is that dust particles can be extremely small, for example 1 to 5 micrometers in diameter. No known inspection systems utilizing radiation scattering are able to detect particles of this size in a high noise environment.
Another problem unique to the inspection of reticles for dust particles is that it is highly desirable to have a system which is insensitive to particles with dimensions less than some predetermined value. No known system provides a measurement of the particle size, particularly one sensitive enough to distinguish between extremely small particles varying in size by only one or two micrometers. The detection of dust on a reticle is also complicated because the intensity of the scattered radiation varies as a function of both the scattering angle and the particle size so that there is no particular angle which is reliably associated with a maximum or minimum degree of scattering.
It is therefore a principal object of this invention to provide a system for automatically detecting minute particles such as dust on a large-area, flat, and optically unpolished surface, such as a glass reticle plate, with a high degree of reliability.
Another object of the invention is to provide such a system which can be adjusted to detect only particles of at least a predetermined size.
A further object of the invention is to provide a dust particle detection system with the foregoing advantages that also is capable of identifying the location of a detected dust particle on the plate.
Another object is to provide a system with the foregoing advantages that can simultaneously examine both faces of a plate.
Yet another object of the invention is to provide a dust particle detection system with the foregoing advantages which can perform an inspection of a reticle plate immediately before it is seated on the optical stage of a projector.
A still further object of this invention is to provide a system with the foregoing advantages that can be used in a controlled atmosphere to reduce the likelihood of contamination of the plate following the inspection.